Surface to Borehole, Cross Borehole Geophysical Methods
Surface to borehole and cross borehole geophysical methods combine the use of
electrodes or geophones in boreholes with surface electrodes or sources to affect surface to
borehole and cross borehole measurements (Dobecki and Romig, 1985). Application of both
surface and borehole geophysical techniques increases the resolution of targets because
borehole probes can be positioned close to the target of interest (Van Nostrand and Cook,
1966). Geophysical techniques applied within and between boreholes include vertical seismic
profiling, geotomography (utilizing both seismic and EM waves) and DC resistivity. Cross
borehole EM techniques have been used by Lytle et al. (1979, 1981) to locate high contrast
electrical anomalies (e.g., tunnels) and to monitor the direction and flow rate of injected
fluids. Butler and Curro (1981) have described cross borehole procedures for obtaining
accurate seismic velocity profiles.
Cross borehole and surface to borehole methods provide a greater lateral radius of
investigation than can be achieved through single borehole logging, thereby providing
measurement over a larger formation volume. The region surveyed is a path between the
energy source and the detector, but it is not necessarily the straight line path between the two
points. The probability that the sampled region is along a straight line path between the
source and detector increases as the distance between the source and detector decreases. The
surface to borehole and cross borehole techniques are limited by many of the factors affecting
most geophysical surveys such as non uniqueness of results, and therefore, require other
integrated data for verification of results.
Characterizing Ground Water Flow Beneath the Site
In addition to characterizing site geology, the owner/operator should characterize the
hydrology of the uppermost aquifer and its confining layer(s) at the site. The owner or
operator should install wells and/or piezometers to assist in characterizing site hydrology.
The owner/operator should determine and assess:
The direction(s) and rate(s) of ground water flow (including both horizontal
and vertical components of flow);
Seasonal/temporal, natural, and artificially induced (e.g., off site production
well pumping, agricultural use) short term and long term variations in ground 
water elevations and flow patterns; and
The hydraulic conductivities of the stratigraphic units at the site, including
vertical hydraulic conductivity of the confining layer(s).
Section 4.3.1 provides a brief introduction to ground water flow in porous media and
conduits; Section 4.3.2 provides a discussion of the Agency's definition of "uppermost
November 1992
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